Manasevit, in U.S. Pat. Nos. 4,404,265 and 4,368,098, describes a basic organometallic chemical vapor deposition (OMCVD) process for forming thin films of single crystal Group III-V wide band-gap semiconductor compounds or alloys on single crystal heated substrates. In general, OMCVD involves the pyrolysis of metal alkyls containing a Group III constituent with hydrides containing a Group V constituent. For example, alkyl metal organics or organo-metallics, such as trimethylgallium (TMG) and/or triethylgallium (TEG) are reacted with a Group V hydride, such as arsine or phosphine or stibnine, to form a GaAs or GaP or GaSb film on a suitable substrate.
Manasevit also describes the background for the development of OMCVD processes. As microelectronic technology evolved, the search for better semiconductor materials became more intense. In the past, most process technology concentrated on the use of single element germanium or silicon for the formation of semiconductor devices. More recently, other semiconductor materials, such as gallium arsenide, have became increasingly important. Gallium arsenide (GaAs) and other III-V semiconductor compounds and alloys have been found to be among the most versatile of all semiconductor materials. For example, varactors, transistors, microwave diodes, light-emitting diodes, injection lasers, bulk microwave power sources, negative resistance amplifiers, and bulk-effect integrated circuits are all possible with gallium arsenide.
Although GaAs, Ge and Si all exhibit semiconducting properties, the differences between GaAs and the elemental semiconductors Ge and Si enhance gallium arsenide's potential usefulness. In GaAs, the minimum of the conduction band and the maximum of the valence band are such that direct electronic transitions can occur between the bands, allowing gallium arsenide to be used, for example, as an injection laser. This is not true for silicon or germanium.
Gallium arsenide has a higher electron mobility and a wider band gap than either germanium or silicon. Furthermore, GaAs has two valleys in its conduction band, separated by an energy difference. Electrons in the lower-energy valley have a higher mobility than those in the higher-energy valley. As the voltage across a sample of GaAs is increased, more electrons are excited into the upper, lower mobility valley and the current decreases, causing bulk instabilities such as the Gunn effect. This permits GaAs to be used for microwave power sources of types not possible for either silicon or germanium.
In the past, the primary obstacle to more extensive exploitation of GaAs has been its relative impurity compared with either germanium or silicon. Because of this impurity problem, high quality transistors could not be fabricated with previously available GaAs materials.
Two methods of crystal growth, the Czochralski and the horizontal Bridgman, had been used to grow bulk single crystal GaAs of relatively high purity from a melt. However, to obtain optimum purity, device-grade, gallium arsenide, growth from the vapor phase is a preferred method.
Attempts to grow GaAs in the vapor phase prior to OMCVD had various shortcomings. First, the techniques required the use of reaction chambers in which two or more regions of the chamber are heated to different, closely controlled temperatures. Such a multiple temperature requirement is difficult to implement in a production facility. Further, the prior art techniques each require use of gallium metal as a source material present in the deposition chamber. Gallium metal is difficult to obtain free of impurities because of its reactivity at high temperature with its container, and these impurities tend to vaporize in the chamber and contaminate the deposited GaAs film.
Two general types of OMCVD reactors have been developed; horizontal and vertical reactors. High quality thin GaAs epilayers have been grown in small volume, laboratory conditions using horizontal cold-walled reactors in which a substrate is mounted on a tilted heated body, i.e., a susceptor, within an elongated quartz cylinder. The longitudinal axis of the cylinder is disposed perpendicular to the gravitational vector to form a horizontal reactor. The susceptor is heated by R.F. induction and a flow of organo-metallic reactants and hydrides is pumped through the reaction chamber. ["A New Method for the Growth of GaAs Epilayer at Low H.sub.2 Pressure", Duchemin et al., Journal of Crystal Growth 45 (1978) pp. 181-186.]
A vertical reactor is shown in U. S. Pat. No. 4,368,098 to Manasevit. This reactor is a conventional "flow-down" vertical OMCVD reactor in which gaseous reactants flow downwardly past an R.F. heated susceptor, upon which a substrate is mounted.
A less conventional vertical "chimney" type reactor is described by Leys et al., in "Growth of Multiple Thin Layer Structures in the GaAs-AlAs System Using a Novel VPE Reactor", Journal of Crystal Growth 68 (1984) pp. 431-436. In the "chimney" type vertical reactor, the reactant gasses flow upwardly in the direction of the longitudinal axis of the reaction chamber which is parallel to the gravitational field vector. In Leys et al., reaction gasses are introduced at the bottom of the chamber and pass through a single hollow, vertical, pipe-shaped susceptor of rectangular cross-section. Substrates are attached to a pair of opposing inner walls of the susceptor. The susceptor is formed of graphite and heated by RF induction.
The conventional "flow down" reactor, as represented by Manasevit's vertical reactor, and the horizontal type reactor of Duchemin et al. suffer from serious thermal instability problems. As the gasses heat up next to the susceptor, they expand. The resulting lowering of density creates a buoyancy effect in which the gas tends to rise in a direction contrary to the desired flow. The resultant flow is not conducive to the formation of abrupt interfaces.
In Ley et al's vertical flow-up system, this buoyancy effect helps to stabilize the gas flow and enables the reactor to grow layers with very abrupt interfaces. This is an advantageous effect for high electron mobility transistors, heterojunction bipolar transistors, and heterostructure devices, in general. However, Leys et al's reactor also produces horizontal thermal gradients. These thermal gradients occur because the bulk of the reactant gasses flow up the cooler central core of the reactor. The resultant flow non-uniformities may be the cause of the large lateral non-uniformity in growth that has been reported by Leys et al.
Attempts to scale-up laboratory OMCVD reactors, be they vertical or horizontal types have met with difficulties, chiefly a loss of uniformity of growth resulting from the increased length of the reaction zone. Efforts to compensate for such non-uniformity have included rotation of the substrate about an axis which is parallel or closely parallel to the flow of reactant gas, or narrowing of the gas flow channel in the longitudinal direction. The former effort fails to address longitudinal non-uniformity due to reactant gas depletion. The latter effort is only practical for short lengths of growth regions and is therefore unsuitable for scale-up.